1. FIELD OF THE INVENTION
This invention relates generally to a discharge machining apparatus, and more particularly to a discharge machining apparatus in which the voltage applied across an electrode and a workpiece is supplied via a main transistor controlled by a pulse signal; the discharge machining apparatus adapted to positively and quickly absorb the energy released by an inductance on a power feeding path when the main transistor is turned off.
2. DESCRIPTION OF THE PRIOR ART
In a discharge machining apparatus, an electric discharge is generated by applying a voltage across an electrode and a workpiece to machine the workpiece by means of the generated discharge energy. In such a discharge machining apparatus, when a voltage is continuously impressed across the electrode and the workpiece, a so-called concentrated discharge takes place, leading to an unwanted discharge machining state. To cope with this, an arrangement is generally employed in which the voltage is applied across the electrode and the workpiece in a pulse voltage mode by means of a switching transistor.
FIG. 9 shows a circuit configuration of a discharge machining apparatus having such a construction. In the figure, reference numeral 1 refers to a main transistor; lB to the base terminal of the main transistor 1; 2 to an electrode; 3 to a workpiece; 4 to a d-c power supply; 5 to a resistor; 7 to a power feeding path; and 41 is a diode, respectively. As is well known, the operation of this circuit is such that pulse voltages are fed across the electrode 2 and the workpiece 3 by the action of the main transistor 1 that turns on and off in accordance with the pulse signals fed to the base terminal 1B.
During actual discharge machining, however, a phenomenon called pulse interruption may occur, in which discharge is instantaneously interrupted and then restored immediately due to a change in a discharge gap between the electrode 2 and the workpiece 3, for example. This pulse interruption tends to occur when the machining surface is too large, or the capacitance of a power feeding path is too high, or under machining conditions as found in finish machining, where the peak value of current: is too low and the discharge gap is too small, relative to the machining area. The pulse interruption causes the electrode to wear rapidly.
To cope with this, it has heretofore been practiced that an inductor 6 is added to a power feeding path 7 having such a circuit configuration as shown in FIG. 9 so that the pulse interruption is avoided by releasing the energy (Li.sup.2 /2 stored in the inductor 6 by means of a newly added diode 42, as shown in FIG. 10. The voltage and current waveforms obtained in the circuit configuration shown in FIG. 10 are shown in FIG. 11. In FIG. 11, .circle.1 is the voltage waveform of a pulse signal fed to the base terminal 1B; .circle.2 the voltage waveform across the emitter and collector of the main transistor 1; .circle.3 the voltage waveform across the electrode 2 and the workpiece 3; and .circle.4 the current waveform of the current i shown in FIG. 10.
As shown in the figure, after the main transistor 1 has been turned off, the energy stored in the inductor 6 is released relatively rapidly by a current flowing via the the current flowing via the diode 42 assumes a damping mode while oscillating at a frequency determined by the circuit resistance. Consequently, the current actually keeps flowing without settling to "zero" due to the oscillation. This brings about a state close to short circuiting between the electrode Z and the workpiece 3 during discharge machining; if the energy (Li.sup.2 /2 stored in the inductor 6 is increased as the peak value of discharge current increases, the current released by the inductor 6 cannot subside completely before the next pulse signal is input, causing voltages to be successively applied across the electrode 2 and the workpiece 3. This could lead to the occurrence of a concentrated discharge. The concentrated discharge may disrupt discharge machining, resulting in lowered machining speed, or the deteriorated accuracy of the machining surface, or an electrode breakage in the wire discharge machining apparatus.
As a method for solving this serious problem, it has been conceived that the current released by the inductor 6 is caused to subside completely by providing a longer time before the next pulse signal is input. This method, however, may give rise to deteriorated machining efficiency.
This invention has been conceived in the light of these circumstances, and is intended to provide a discharge machining apparatus that is capable of positively and rapidly absorbing the energy released by the inductance of a power feeding path when a main transistor is turned off.